1. Field of the Invention
The present invention relates to a communication terminal out of range determination method, which is suitable for determining whether or not a communication terminal is within communicable range.
2. Description of Related Art
With the recently developed Code Division Multiple Access (CDMA) communication method, a base station receives code division multiplexed data transmitted from a mobile terminal such as a portable telephone, and performs despread processing or the like on the received code division multiplexed data, thereby obtaining data on mobile terminals used by individual users. Here, because data transmitted from different mobile terminals uses the same frequency band, this results in interference waves. The degree of interference depends on the transmission power level of the other mobile terminals, and increases as the transmission power levels of other mobile terminals increase. Furthermore, the signal level at the time when the base station receives data transmitted from a mobile terminal, varies according to the distance and geographical features and the like between the base station and the mobile terminal.
In order to solve such problems, the base station instructs each mobile terminal to notify the base station of the received signal level (power level) of the downstream signal transmitted from the base station to the mobile terminals, and controls transmission power so that the signal level of all received data is the same. This transmission power control is described below.
The base station monitors the received signal level (power level) of the downstream signals of each mobile terminal, transmitted from each mobile terminal. If the received downstream signal level falls below a constant, the base station determines that the state of communication between a given mobile terminal and the base station is impaired (for example, because of long distances between the mobile terminal and the base station, the mobile terminal being used behind a shielding object, or an increase in interference).
In order to stay in communication with that mobile terminal, the base station then issues an instruction to the mobile terminal to increase transmission output level. Upon receiving this instruction, the mobile terminal raises its transmission signal output level (transmission power level) by one level, for example, and tries to stay in communication with the base station. The continuation of this exchange between the base station and the mobile terminal constitutes the transmission power control.
Furthermore, the mobile terminal monitors the received signal level for when data transmitted from the base station is received, and determines the state of communication between the base station and the mobile terminal, based on this received signal level. The mobile terminals then displays the thus determined state of the communication on a screen in the form of an antenna symbol, for example, to notify the user of the state of the communication.
Furthermore, if the mobile terminal receives data transmitted from the base station at a received signal level which is below a certain threshold, then the mobile terminal determines that the mobile terminal cannot stay in communication with the base station, that is, the mobile terminal is out of range. The mobile terminal then displays on the screen that the mobile terminal is now out of range.
The matters described above are disclosed in Japanese Unexamined Patent Application, First Publication No. 2000-196519.
Incidentally, if the so-called upstream communication method for when the data is transmitted from the mobile terminal to the base station, and the so-called downstream communication method for when the data is transmitted from the base station to the mobile terminal are the same, then provided that the receiver sensitivity does not differ greatly between the base station and the mobile terminal, the conventional out of range determination method above can be used to determine with a degree of precision when a mobile terminal is out of range.
However, in this conventional out of range determination method, if the upstream and downstream communication methods differ, then for each communication method the characteristics such as strength and reflection differ with respect to radio interference and shielding materials. For example, if the downstream method is a Time Division Multiple Access (TDMA) communication method and the upstream method is the CDMA communication method, then there is the possibility that although the signal of the downstream TDMA method is able to reach the mobile terminal from the base station, the signal of the upstream CDMA method is not able to reach the base station from the mobile terminal. Therefore, in such a case, communication may not be able to be maintained between the mobile terminal and the base station. Also if the downstream method is the CDMA communication method and the upstream method is the TDMA communication method, the same would apply, namely that communication cannot be achieved between the mobile terminal and the base station.
In other words, with conventional communication methods, if the communication methods for upstream and downstream differ, then it is impossible to determine accurately whether a mobile terminal is out of range based only on the receiver sensitivity (downstream signal reception level) of the mobile terminal.